For background, reference is made to U.S. Pat. Nos. 5,870,484, 5,333,200, 4,569,074, “Gradient Loudspeakers,” Harry F. Olson, Journal of the Audio Engineering Society, Loudspeaker Anthology, Vol. 1, p. 304, and “Spatial Hearing,” Jens Blauert, 2nd Ed., 1983, MIT Press, Cambridge, Mass.